Connection pool can discard broken connections during acquisition
(when connection liveness check timeout is configured) and when
connections are returned to the pool. In both cases connections
should be disposed and removed from the set of active connections.
This is especially important with least connected load balancing
strategy which examines amount of active connections for each address.

This commit makes sure broken connections are disposed through the
connections queue to make sure active set is always updated.

Require a null forward merge to 1.5